Cerebral Arteriolar Structure in Mice Overexpressing
Human Renin and Angiotensinogen
Gary L. Baumbach, Curt D. Sigmund, Frank M. Faraci
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Abstract—We examined the hypothesis that the renin-angiotensin system plays an important role in vascular remodeling
(defined as reduced external diameter) during chronic hypertension. We measured pressure, diameter, and crosssectional area of the vessel wall in maximally dilated cerebral arterioles in transgenic mice that overexpress both human
renin and human angiotensinogen and in spontaneously hypertensive mice, a model of chronic hypertension that is
thought to develop independently of the renin-angiotensin system. Systemic arterial pressure under conscious conditions
was increased by similar amounts in transgenically hypertensive mice (153⫾6 versus 117⫾4 mm Hg in controls;
mean⫾SE, P⬍0.05) and spontaneously hypertensive mice (148⫾5 versus 112⫾5 mm Hg; P⬍0.05). The external
diameter of maximally dilated cerebral arterioles was reduced in transgenically hypertensive mice (52⫾2 versus 66⫾3
␮m; P⬍0.05), but not in spontaneously hypertensive mice (58⫾4 versus 60⫾4 ␮m; P⬎0.05). The cross-sectional area
of the vessel wall was increased in both transgenically hypertensive mice (504⫾53 versus 379⫾37 ␮m2; P⬍0.05) and
spontaneously hypertensive mice (488⫾40 versus 328⫾38 ␮m2; P⬍0.05). During maximal dilatation, the stress-strain
curves in cerebral arterioles of transgenically hypertensive mice and spontaneously hypertensive mice were shifted to
the right of the curves in corresponding controls, an indication that arteriolar distensibility was increased in the
transgenically and spontaneously hypertensive groups. Thus, cerebral arterioles undergo remodeling and hypertrophy in
transgenically hypertensive mice, but only hypertrophy in spontaneously hypertensive mice. These findings support the
hypothesis that the renin-angiotensin system is an important determinant of vascular remodeling during chronic
hypertension. (Hypertension. 2003;41:50-55.)
Key Words: renin-angiotensin system 䡲 mice 䡲 vasculature 䡲 remodeling 䡲 hypertrophy 䡲 hypertension, chronic
C
modeling of cerebral arterioles in SHRSP.10,11 Because the
ACE inhibitors lowered arterial pressure in SHRSP more
effectively than hydralazine and propranolol, however, we
were unable to draw definitive conclusions from these studies
with regard to the direct effects of the ACE inhibitor on
cerebral vascular remodeling and hypertrophy, as opposed to
the effects of arterial pressure per se.
A major goal of this study was to examine further the
hypothesis that the renin-angiotensin system plays an important role in vascular remodeling during chronic hypertension.
To accomplish this goal, we examined structural characteristics of cerebral arterioles in 2 experimental models of
hypertension. Transgenic mice that overexpress both human
renin and human angiotensinogen (R⫹/A⫹)12–14 are a novel,
defined model of hypertension. The other model was the
BPH-2 mouse, a model of chronic hypertension that is
thought to represent a non–renin-dependent mode of hypertension.15 We anticipated that if the renin-angiotensin system
is an important determinant of vascular remodeling, cerebral
arterioles would show evidence of remodeling in R⫹/A⫹
mice but not in BPH-2 mice.
hronic hypertension alters the structure and mechanics of
cerebral arterioles. Cerebral arterioles in spontaneously
hypertensive rats (SHR) and stroke-prone SHR (SHRSP)
undergo both hypertrophy of the vessel wall and reduction in
external diameter (remodeling).1,2 Despite hypertrophy, cerebral arterioles undergo a paradoxical increase in passive
distensibility of the vessel wall during chronic hypertension
in SHRSP.3 Because these alterations may contribute to the
increased risk of stroke that accompanies chronic hypertension, it is important to examine determinants of these alterations in the cerebral circulation, as opposed to other vascular
beds, so that we may better understand the link between
chronic hypertension and stroke.
Alterations in vascular structure during chronic hypertension may result from a number of determinants, including
arterial pressure,4 neurohumoral factors,5,6 and endotheliumderived factors.7–9 A determinant of particular interest with
respect to vascular remodeling has been the renin-angiotensin
system. This interest was stimulated by previous studies in
which we found that angiotensin-converting enzyme (ACE)
inhibitors, but not hydralazine or propranolol, attenuate re-
Received June 5, 2002; first decision July 1, 2002; revision accepted October 7, 2002.
From the Departments of Pathology (G.L.B.) and Internal Medicine (C.D.S., F.M.F.), University of Iowa College of Medicine and Cardiovascular
Center, Iowa City, Iowa.
Correspondence to Gary L. Baumbach, MD, Department of Pathology, 105 Medical Laboratories, University of Iowa College of Medicine, Iowa City,
IA 52242. E-mail [email protected]
© 2003 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
DOI: 10.1161/01.HYP.0000042427.05390.5C
50
Baumbach et al
Methods
Animals
The experimental protocol was approved by our institution’s animal
care and use committee. All breeding and genotyping was performed
in the transgenic animal facility (directed by C.D.S.), located in a
virus- and pathogen-free animal care facility.
Transgenic Mice
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Double transgenic mice (R⫹/A⫹) were generated by crossbreeding
human renin (R⫹) mice with human angiotensinogen (A⫹) mice, as
we have reported previously.13,14 The presence of the transgenes was
assessed by gene- and species-specific polymerase chain reaction of
DNA isolated from tail biopsy samples, as described previously.13,14
There are no differences in blood pressure between R⫺/A⫺ and
single transgenic mice (R⫹/A⫺ or R⫺/A⫹) owing to the strict
species specificity in the enzymatic reaction between renin and
angiotensinogen.13 Because of this specificity, mouse renin does not
cleave human angiotensinogen and human renin does not cleave
mouse angiotensinogen.13 Because blood pressure is the same in all
3 mice, R⫺/A⫺, R⫹/A⫺, and R⫺/A⫹ mice were all used as
controls in the present study. Control (n⫽11) and R⫹/A⫹ (n⫽12)
mice averaged 6.5 months of age. The body weights of control and
R⫹/⫹ mice were 28⫾5 and 24⫾4 g, respectively.
BPH-2 Mice
Spontaneously hypertensive BPH-2 mice were maintained in the
animal care facility. The BPH-2 mouse colony was established in
Iowa by brother-sister mating of several breeding pairs obtained
from the colony at the University of Kansas. The original derivation
of the BPH-2 strain has been described previously.16,17 C57BL/6
mice (Harlan, Indianapolis, Ind) were used as controls. Control
(n⫽10) and BPH-2 (n⫽10) mice averaged 7.5 months of age. The
body weights of control and BPH-2 mice were 30⫾3 and 26⫾4 g,
respectively.
Systemic Arterial Pressure in Conscious Mice
We and others have found that the most accurate measurements of
arterial pressure in mice are obtained with chronic indwelling
catheters in conscious animals. We therefore measured systemic
arterial pressure in conscious mice using a method described
previously.13 For chronic catheterization, mice were anesthetized
with Avertin (0.2 to 0.3 mL, IP), shaved, and prepped with a 70%
alcohol solution. Sterile catheters (0.040 in outer diameter⫻0.025 in
inner diameter) were placed in the right carotid artery with the aid of
a dissecting microscope. Mice were placed on a warming pad (39°C)
during the surgical procedure and postoperatively until fully awake.
All animals were given prophylactic antibiotics (penicillin G, 12 000
U, IM) and allowed to recover at least 24 hours before measuring
systemic arterial pressure under conscious conditions.
In Vivo Preparation
Animals were weighed and anesthetized with sodium pentobarbital
(5 mg per 100 g body weight [BW], IP), intubated, and mechanically
ventilated with room air and supplemental O2. Additional anesthesia
(1.7 mg per 100 g BW, IV) was administered when pressure to a paw
evoked a change in blood pressure or heart rate.
A catheter was inserted into a femoral vein for injection of drugs
and fluids. Catheters were inserted into both femoral arteries to
record systemic arterial pressure, obtain blood samples for measurement of arterial blood gases, and withdraw blood to produce
hypotension (needed for studies of vascular mechanics).
Measurements in Cerebral Arterioles
We measured pressure and diameter in first-order arterioles on the
cerebrum through an open skull preparation as described previously.3
A craniotomy was made over the left parietal cortex, and the dura
was incised to expose cerebral vessels. Exposed brain was continuously suffused with artificial cerebrospinal fluid (CSF) warmed to 37
to 38°C and equilibrated with a gas mixture of 5% CO2 and 95% N2.
Systolic, diastolic, mean, and pulse pressures were measured con-
Vascular Structure in Renin-Angiotensinogen Mice
51
tinuously in cerebral arterioles with a servo-null pressure-measuring
device (model 5, Instrumentation for Physiology & Medicine, Inc).
Arterioles were monitored through a microscope connected to a
closed-circuit video system with a final magnification of ⫻356.
Arteriolar diameter was measured from digitized images using image
analysis software (NIH Image, National Institutes of Health, Research Services Branch, NIMH). Cross-sectional area of the arteriolar wall was determined histologically from 1-␮m sections using a
light microscope interfaced with a video image analyzing system.
Circumferential stress, circumferential strain, and tangential elastic
modulus were calculated from measurements of cerebral arteriolar
pressure, diameter, and cross-sectional area of the vessel wall as
described previously.3
Experimental Protocol
After obtaining measurements under baseline conditions, cerebral
arterioles were suffused with artificial CSF containing EDTA
(67 mmol/L) to deactivate vascular smooth muscle.3 Pressurediameter relationships were then obtained in maximally dilated
cerebral arterioles. Hemorrhage was used to reduce cerebral arteriolar pressure in decrements of 10 mm Hg at pressures down to
20 mm Hg and decrements of 5 mm Hg at pressures between 20 and
10 mm Hg. Maximally dilated arterioles were fixed at physiological
pressure in vivo by suffusion of vessels with glutaraldehyde fixative
(2.25% glutaraldehyde in 0.10 mol/L cacodylate buffer). After the
animal was killed by an injection of potassium chloride, the arteriolar
segment used for pressure-diameter measurements was removed,
processed, and embedded in Spurr’s low viscosity resin while
cross-sectional orientation was maintained.
Statistical Analysis
Analysis of variance was used to compare systemic mean pressure,
arteriolar pressures, diameters, cross-sectional area of the vessel
wall, cross-sectional area and volume density of individual components, ratios of nondistensible to distensible components, and slope
of tangential elastic modulus versus stress. Probability values were
calculated using a Student t test. Statistics were determined using
JMP statistics software (SAS Institute Inc) on a Macintosh computer.
Results
Renin-Angiotensinogen Mice
Baseline Values
Systemic arterial mean pressure was significantly greater in
R⫹/A⫹ mice than in control mice in both the conscious and
anesthetized states, even though anesthesia significantly reduced systemic arterial mean pressure in both groups (Table
1). Cerebral arteriolar systolic, diastolic, mean, and pulse
pressures were significantly greater in R⫹/A⫹ mice than in
control mice (Table 1). Diameter before deactivation with
EDTA was not significantly different in cerebral arterioles in
R⫹/A⫹ mice from that in control mice (Table 1). During
deactivation with EDTA, internal and external diameters
were significantly less in cerebral arterioles in R⫹/A⫹ mice
than in control mice (Table 1). Dilator reserve (defined as the
difference in internal diameter of cerebral arterioles before
and after maximal dilatation) was significantly less in
R⫹/A⫹ mice than in control mice (17⫾3 versus 28⫾3 ␮m;
P⬍0.05). The cross-sectional area of the vessel wall was
significantly greater in cerebral arterioles in R⫹/A⫹ mice
than in control mice (Table 1). Thus, cerebral arterioles in
R⫹/A⫹ mice underwent hypertrophy and remodeling with a
reduction in external diameter, as well as a reduction in
dilator reserve.
52
Hypertension
TABLE 1.
January 2003
Baseline Values in Renin-Angiotensinogen Mice
Parameter
Controls
Trangenics
Before deactivation of smooth muscle
Systemic arterial mean pressure, mm Hg
Unanesthetized
117⫾4
153⫾6*
88⫾4
113⫾5*
Systolic
48⫾1
74⫾5*
Diastolic
36⫾2
53⫾4*
Mean
40⫾1
60⫾4*
Pulse
12⫾1
21⫾2*
Anesthetized
Cerebral arteriolar pressure, mm Hg
Arterial blood gases
37⫾1
36⫾1
pH
7.34⫾0.04
7.33⫾0.03
pO2, mm Hg
124⫾6
136⫾8
34⫾4
28⫾3
62⫾3
45⫾2*
pCO2, mm Hg
Internal cerebral arteriolar diameter, mm
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After deactivation of smooth muscle
Cerebral arteriolar diameter, mm
Internal
66⫾3
52⫾2*
Cross-sectional area of vessel wall, mm2
External
379⫾37
504⫾53*
ET vs stress
6.0⫾0.9
4.0⫾0.3*
Measurements of internal diameter before deactivation of smooth muscle
were obtained at prevailing levels of arterial pressure. Measurements of
internal diameter after deactivation of smooth muscle were made at an
arteriolar mean pressure of 40 mm Hg. Values of external diameter after
deactivation of smooth muscle were calculated from measurements of internal
diameter at 40 mm Hg arteriolar pressure and histological measurements of
cross-sectional area of the vessel wall. ET vs stress indicates slope of tangential
elastic modulus (ET) versus stress.
Values are mean⫾SEM in 11 control mice and 12 R⫹/A⫹ mice. *P⬍0.05
vs control.
Vascular Mechanics
Internal and external diameters in cerebral arterioles during
maximal dilatation were smaller in R⫹/A⫹ mice than in
control mice at all levels of arteriolar pressure between 10 and
40 mm Hg (Figure 1). The stress-strain curve in cerebral
arterioles of R⫹/A⫹ mice was shifted to the right of the
curve in cerebral arterioles of control mice (Figure 2, left). In
Figure 2. Stress-strain relationship (left) and stress versus tangential elastic modulus (right) in cerebral arterioles during maximal dilatation with EDTA in control (n⫽11) and R⫹/A⫹ mice
(n⫽12). Values are mean⫾SEM. D indicates cerebral arteriolar
diameter; D0, original cerebral arteriolar diameter.
addition, the slope of tangential elastic modulus versus stress
was significantly less in R⫹/A⫹ mice and control mice
(Figure 2, right). These findings suggest that hypertrophy of
cerebral arterioles in R⫹/A⫹ mice was accompanied by an
increase in passive distensibility of cerebral arterioles.
BPH-2 Mice
Baseline Values
Systemic arterial mean pressure was significantly greater in
BPH-2 mice than in control mice in both the conscious and
anesthetized states, even though anesthesia significantly reduced systemic arterial mean pressure in both groups (Table
2). Cerebral arteriolar systolic, diastolic, mean, and pulse
pressures were significantly greater in BPH-2 mice than in
control mice (Table 2). Furthermore, the levels of systemic
arterial mean pressure and cerebral arteriolar pressures in
BPH-2 mice (Table 2) were not significantly different
(P⬎0.05) from levels observed in R⫹/A⫹ mice (Table 1).
The internal diameter before deactivation with EDTA and
internal and external diameters after deactivation with EDTA
were not significantly different in cerebral arterioles of
BPH-2 mice and control mice (Table 2). Dilator reserve was
not significantly different in BPH-2 mice from that in control
mice (25⫾2 versus 22⫾2 ␮m; P⬎0.05). The cross-sectional
area of the vessel wall was significantly greater in cerebral
arterioles in BPH-2 mice than in control mice (Table 2).
Thus, cerebral arterioles in BPH-2 mice underwent hypertrophy, but not remodeling or a reduction in dilator reserve.
Vascular Mechanics
Internal and external diameters in cerebral arterioles during
maximal dilatation were similar in BPH-2 mice and control
mice at all levels of arteriolar pressure between 10 and
40 mm Hg (Figure 3). The stress-strain curve in cerebral
arterioles in BPH-2 mice was shifted to the right of the curve
in cerebral arterioles in control mice (Figure 4, left). In
addition, the slope of tangential elastic modulus versus stress
was significantly less in BPH-2 mice and control mice
(Figure 4, right). These findings suggest that distensibility
was increased in the cerebral arterioles of BPH-2 mice.
Figure 1. Pressure-diameter relationships (internal diameter [left]
and external diameter [right]) in cerebral arterioles during maximal dilatation with EDTA in control (n⫽11) and R⫹/A⫹ mice
(n⫽12). Values are mean⫾SEM. *P⬍0.05 versus control..
Discussion
There are several new findings in this study. First, cerebral
arterioles in R⫹/A⫹ mice undergo remodeling, with a
Baumbach et al
TABLE 2.
Vascular Structure in Renin-Angiotensinogen Mice
53
Baseline Values in BPH-2 Mice
Parameter
Controls
BPH-2
Before deactivation of smooth muscle
Systemic arterial mean pressure, mm Hg
Unanesthetized
112⫾5
148⫾5*
86⫾5
110⫾6*
Systolic
53⫾2
76⫾7*
Diastolic
41⫾3
57⫾6*
Mean
45⫾2
63⫾6*
Pulse
12⫾2
19⫾1*
Anesthetized
Cerebral arteriolar pressure, mm Hg
Arterial blood gases
32⫾2
34⫾2
pH
7.36⫾0.02
7.35⫾0.03
pO2, mm Hg
113⫾8
119⫾10
32⫾4
29⫾2
55⫾3
54⫾4
pCO2, mm Hg
Internal cerebral arteriolar diameter, mm
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After deactivation of smooth muscle
Cerebral arteriolar diameter, mm
Internal
58⫾4
60⫾4
Cross-sectional area of vessel wall, mm2
External
328⫾38
488⫾40*
ET vs stress
5.8⫾0.3
4.1⫾0.4*
Measurements of internal diameter before deactivation of smooth muscle
were obtained at prevailing levels of arterial pressure. Measurements of
internal diameter after deactivation of smooth muscle were made at an
arteriolar mean pressure of 40 mm Hg. Values of external diameter after
deactivation of smooth muscle were calculated from measurements of internal
diameter at 40 mm Hg arteriolar pressure and histological measurements of
cross-sectional area of the vessel wall. ET vs Stress: slope of tangential elastic
modulus (ET) versus stress.
Values are mean⫾SEM in 10 control mice and 10 BPH-2 mice. *P⬍0.05 vs
control.
reduction in external diameter, as well as hypertrophy and
increased distensibility of the vessel wall. The finding of
remodeling in cerebral arterioles of R⫹/A⫹ mice suggests
that the renin-angiotensin system may contribute to remodeling of cerebral arterioles during chronic hypertension.
Second, cerebral arterioles in BPH-2 mice, an experimental
model of hypertension thought to be renin-independent,
Figure 3. Pressure-diameter relationships (internal diameter [left]
and external diameter [right]) in cerebral arterioles during maximal dilatation with EDTA in control (n⫽10) and BPH-2 mice
(n⫽10). Values are mean⫾SEM.
Figure 4. Stress-strain relationship (left) and stress versus tangential elastic modulus (right) in cerebral arterioles during maximal dilatation with EDTA in control (n⫽10) and BPH-2 mice
(n⫽10). Values are mean⫾SEM.
undergo hypertrophy with an increase in distensibility, but do
not undergo remodeling. These findings suggest that increases in arterial pressure per se are not sufficient to induce
remodeling of cerebral arterioles. More importantly, the
findings provide additional support for the hypothesis we
proposed previously that the renin-angiotensin system may
play an important role in cerebral vascular remodeling during
chronic hypertension. In addition to these new findings and
their implications regarding the renin-angiotensin system, this
study is the first to measure arteriolar/microvascular pressure
in any vascular bed in mice. Cerebral arteriolar mean pressure
in normotensive mice was about 45% to 55% of systemic
arterial mean pressure, a pressure reduction that is comparable to the drop in pressure between aorta and cerebral
arterioles found in normotensive rats.5
Remodeling
We define remodeling as a reduction in external diameter of
small resistance arteries and arterioles during chronic hypertension that cannot be attributed to altered distensibility of the
vessel wall.1 Determinants of vascular remodeling during
chronic hypertension are not yet well understood. Based on
previous findings, we have proposed that the renin-angiotensin system may be a determinant of remodeling. In one study,
we found that the ACE inhibitor, cilazapril, attenuated remodeling in cerebral arterioles in SHRSP, in contrast to
hydralazine, which had no effect on cerebral arteriolar remodeling.10 We then observed in a subsequent study that
remodeling of cerebral arterioles in SHRSP was attenuated
nearly as effectively by a low dose of perindopril as by a high
dose, even though the low dose of perindopril was half as
effective as the high dose in lowering cerebral arteriolar
pressure.11 In that same study,11 we also found that, in
contrast to the low dose of perindopril, the ␤-adrenergic
receptor blocker, propranolol, did not significantly attenuate
remodeling of cerebral arterioles in SHRSP, even though it
was much more effective than the low dose of perindopril in
lowering cerebral arteriolar pressure.
We also, however, recognize the limitations of these
studies.10,11 First, because hydralazine was significantly less
effective than cilazapril in lowering arterial pressure in
SHRSP, we were unable to unambiguously rule out the
possibility that the effects of cilazapril on cerebral arteriolar
54
Hypertension
January 2003
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remodeling were secondary to reductions in arterial pressure
rather than to direct effects of ACE inhibition.10 Second,
treatment with propranolol, but not perindopril, resulted in a
significant reduction in body weight of SHRSP.11 If vessel
size is proportional to body weight, then reduction of body
weight during treatment with propranolol may have contributed to the finding of smaller external diameters in cerebral
arterioles of SHRSP treated with propranolol than in SHRSP
treated with the low and high doses of perindopril. Finally, in
addition to their ability to inhibit conversion of angiotensin I
to angiotensin II, ACE inhibitors also inhibit inactivation of
bradykinins.18 Thus, we cannot rule out the possibility that
cilazapril10 and perindopril11 attenuated cerebral arteriolar
remodeling in SHRSP by increasing availability of bradykinins rather than decreasing availability of angiotensin II.
We undertook the present study in an effort to further
define the contributions of the renin-angiotensin system to
cerebral arteriolar remodeling. The R⫹/A⫹ transgenic mouse
is a well-defined model of angiotensin II–induced chronic
hypertension. In addition, the genetic background of R⫹/A⫹
transgenic mice is nearly identical to that of the control
animals because the mice used in these studies were derived
from 6 to 7 generations of back-crossbreeding to the C57BL/
6J. Because previous studies examining effects of hypertension in SHRSP versus Wistar Kyoto rats have used strains
that are genetically diverse, the results are clouded by the
presence of genes in the genetic background that may
themselves promote vascular remodeling.
We found in this study that cerebral arterioles in R⫹/A⫹
mice undergo significant remodeling of the vessel wall. In
contrast, cerebral arterioles in BPH-2 mice do not undergo
remodeling. These findings provide additional support for the
hypothesis that the renin-angiotensin system is a major
determinant of vascular remodeling during chronic hypertension. The basis for this statement is that hypertension in
R⫹/A⫹ results directly from overexpression of human renin
and angiotensinogen,13,14 whereas hypertension in BPH-2
mice is thought to be independent of the renin-angiotensin
system.15 Levels of tissue and plasma renin and renin activity
are not significantly different in adult and juvenile BPH-2
mice relative to normotensive controls.15
Hypertrophy
Determinants that may contribute to vascular hypertrophy
during chronic hypertension include increases in arterial
pressure4 and the renin-angiotensin system.6 Perhaps the best
evidence obtained in vivo that supports a direct role for the
renin-angiotensin system is provided by a study in which the
pressor effects of angiotensin II were counteracted by simultaneous treatment with hydralazine.19 The cross-sectional
area of the vessel wall in mesenteric resistance arteries of rats
was increased by chronic infusion of angiotensin II, even
when increases in arterial pressure were prevented by
hydralazine.
The findings in this study do not provide convincing
support for an essential role of the renin-angiotensin system
in hypertrophy of cerebral arterioles. We found that cerebral
arterioles undergo hypertrophy in BPH-2 mice, as well as in
R⫹/A⫹ mice. Increases in systemic arterial mean pressure
and cerebral arteriolar mean and pulse pressures were similar
in the 2 groups of mice. Levels of renin-angiotensin activity,
on the other hand, were presumably quite different. Whereas,
activity of the renin-angiotensin system is known to be
elevated in R⫹/A⫹ mice,13,14 activity in BPH-2 mice is
thought to be normal.15 Thus, although increased activity of
the renin-angiotensin system may contribute to the development of cerebral arteriolar hypertrophy in R⫹/A⫹ mice, it is
likely that other factors, such as increases in arteriolar
pressure, endothelial factors, or sympathetic nerves, play a
more important role than renin-angiotensin in hypertrophy of
cerebral arterioles in BPH-2 mice.
Vascular Mechanics
The distensibility of fully relaxed cerebral arterioles is increased paradoxically in SHRSP, SHR, and rats with
1-kidney, 1-clip renal hypertension, despite hypertrophy of
the arteriolar wall.2,3 Furthermore, prevention of hypertrophy
in cerebral arterioles of SHRSP by treatment with an ACE
inhibitor10 or carotid clipping20 significantly attenuates increases in arteriolar distensibility. We were not surprised,
therefore, by the finding in this study that distensibility, as
well as the cross-sectional area of the vessel wall, was
increased in cerebral arterioles of R⫹/A⫹ mice and BPH-2
mice.
We have proposed previously that increases in passive
distensibility that accompany hypertrophy of cerebral arterioles may be due to a reduction in the proportion of stiff
(collagen and basement membrane) to compliant (smooth
muscle, elastin, and endothelium) components of the arteriolar wall in cerebral arterioles.2,5,20,21 Therefore, a possible
explanation for the finding in this study of increased cerebral
arteriolar distensibility in R⫹/A⫹ and BPH-2 mice is that
hypertrophy of the vessel wall is accompanied by a disproportionate increase in the more compliant components of the
vessel wall. Another possibility that cannot be ruled out is
that matrix components in the arteriolar wall undergo qualitative alterations during chronic hypertension, which, in turn,
lead to increases in passive distensibility.
Perspectives
We have proposed previously that one of the mechanisms that
could result in remodeling may involve migration of smooth
muscle cells within the vessel wall.22 Angiotensin II has been
shown to stimulate migration of vascular smooth muscle in
tissue culture.23,24 It is also of interest to note that nitric oxide
inhibits angiotensin-stimulated migration of vascular smooth
muscle.25 Angiotensin II also stimulates formation of superoxide,26 which, in turn, deactivates nitric oxide.27 Thus,
angiotensin II may stimulate migration of vascular muscle
directly and/or indirectly through removal of the inhibitory
influence of nitric oxide. Enhanced migration of smooth
muscle in the vessel wall could lead to remodeling with a
reduction in external diameter by enabling adjacent smooth
muscle cell processes to move past each other and increase
the number of times each smooth muscle cell wraps itself
around the vascular lumen (ie, an increase in wrapping
distance). By increasing their wrapping distance, the migration of smooth muscle cells within the vessel wall would, in
Baumbach et al
effect, reduce vascular circumference, thereby reducing external diameter and producing encroachment of the tunica
media into the lumen.
Another implication of this study relates to factors that may
contribute to reductions in dilator reserve during chronic
hypertension. Dilator reserve is defined as the difference in
vessel diameter before and after maximal dilatation. We have
proposed previously that remodeling with a reduction in
external diameter may contribute to impairment of dilator
reserve in the cerebral circulation during chronic hypertension.28 This concept is supported by the findings in this study
that cerebral arterioles in R⫹/A⫹ mice undergo both remodeling and a reduction in vasodilator reserve, whereas cerebral
arterioles in BPH-2 mice undergo neither. Furthermore, the
finding that cerebral arterioles undergo hypertrophy of the
vessel wall in both R⫹/A⫹ and BPH-2 mice suggests that
hypertrophy per se may not play an important role in the
impairment of dilator reserve during chronic hypertension.
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Acknowledgments
We thank Tom Gerhold for technical assistance. This work was
supported by National Institutes of Health grants HL-22149, HL62984, and NS-24621.
References
1. Baumbach GL, Heistad DD. Remodeling of cerebral arterioles in chronic
hypertension. Hypertension. 1989;13:968 –972.
2. Baumbach GL, Hajdu MA. Mechanics and composition of cerebral arterioles in renal and spontaneously hypertensive rats. Hypertension. 1993;
21:816 – 826.
3. Baumbach GL, Dobrin PB, Hart MN, Heistad DD. Mechanics of cerebral
arterioles in hypertensive rats. Circ Res. 1988;62:56 – 64.
4. Folkow B, Gurevich M, Hallbäck M, Lundgren Y, Weiss L. The hemodynamic consequences of regional hypotension in spontaneously hypertensive and normotensive rats. Acta Physiol Scand. 1971;83:532–541.
5. Baumbach GL, Heistad DD, Siems JE. Effect of sympathetic nerves on
composition and distensibility of cerebral arterioles in rats. J Physiol
(Lond). 1989;416:123–140.
6. Fischli W, Hefti F, Clozel J-P. Effects of acute and chronic cilazapril
treatment in spontaneously hypertensive rats. Br J Clin Pharmacol. 1989;
27:151S–158S.
7. Chillon JM, Heistad DD, Baumbach GL. Effects of endothelin receptor
inhibition on cerebral arterioles in hypertensive rats. Hypertension. 1996;
27:794 –798.
8. Li JS, Larivière R, Schiffrin EL. Effect of a nonselective endothelin
antagonist on vascular remodeling in deoxycorticosterone acetate-salt
hypertensive rats: evidence for a role of endothelin in vascular hypertrophy. Hypertension. 1994;24:183–188.
9. Chillon JM, Ghoneim S, Baumbach GL. Effects of nitric oxide inhibition
on mechanics of cerebral arterioles in rats. Hypertension. 1997;30:
1097–1104.
Vascular Structure in Renin-Angiotensinogen Mice
55
10. Hajdu MA, Heistad DD, Baumbach GL. Effects of antihypertensive
therapy on mechanics of cerebral arterioles in rats. Hypertension. 1991;
17:308 –316.
11. Chillon JM, Baumbach GL. Effects of an angiotensin-converting enzyme
inhibitor and a beta-blocker on cerebral arterioles in rats. Hypertension.
1999;33:856 – 861.
12. Merrill DC, Granwehr BP, Davis DR, Sigmund CD. Use of transgenic
and gene-targeted mice to model the genetic basis of hypertensive disorders. Proc Assoc Am Physicians. 1997;109:533–546.
13. Merrill DC, Thompson MW, Carney CL, Granwehr BP, Schlager G,
Robillard JE, Sigmund CD. Chronic hypertension and altered baroreflex
responses in transgenic mice containing the human renin and human
angiotensinogen genes. J Clin Invest. 1996;97:1047–1055.
14. Davisson RL, Yang G, Beltz TG, Cassell MD, Johnson AK, Sigmund
CD. The brain renin-angiotensin system contributes to the hypertension in
mice containing both the human renin and human angiotensinogen
transgenes. Circ Res. 1998;83:1047–1058.
15. Iwao H, Nakamura N, Shokei K, Ikemoto F, Yamamoto K, Schlager G.
Renin-angiotensin system in genetically hypertensive mice. Jpn Circ J.
1984;48:1270 –1279.
16. Schlager G. Selection for blood pressure levels in mice. Genetics. 1974;
76:537–549.
17. Schlager G. Genetic hypertension in mice. In: Ganten D, de Jong W, eds.
Handbook of Hypertension. Amsterdam: Elsevier; 1994:158 –172.
18. Pontieri V, Lopes OU, Ferreira SH. Hypotensive effect of captopril: role
of bradykinin and prostaglandinlike substances. Hypertension. 1990;
15(suppl):I55–I58.
19. Griffin SA, Brown WCB, MacPherson F, McGrath JC, Wilson VG,
Korsgaard N, Mulvany MJ, Lever AF. Angiotensin II causes vascular
hypertrophy in part by a non-pressor mechanism. Hypertension. 1991;17:
626 – 635.
20. Baumbach GL, Siems JE, Heistad DD. Effects of local reduction in
pressure on distensibility and composition of cerebral arterioles. Circ Res.
1991;68:338 –351.
21. Baumbach GL, Walmsley JG, Hart MN. Composition and mechanics of
cerebral arterioles in hypertensive rats. Am J Pathol. 1988;133:464 – 471.
22. Baumbach GL, Heistad DD. Mechanics of cerebral arterioles in chronic
hypertension. In: Halpern W, Pegram BL, Brayden J, MacKay K,
McLaughlin M, Osol G, eds. Proceeding of the Second International
Symposium on Resistance Arteries. Ithaca, NY: Perinatology Press;
1988:355–361.
23. Bell L, Madri JA. Influence of the angiotensin system on endothelial and
smooth muscle cell migration. Am J Pathol. 1990;137:7–12.
24. Greene EL, Lu G, Zhang D, Egan BM. Signaling events mediating the
additive effects of oleic acid and angiotensin II on vascular smooth
muscle cell migration. Hypertension. 2001;37:308 –312.
25. Dubey RK, Jackson EK, Lüscher TF. Nitric oxide inhibits angiotensin
II–induced migration of rat aortic smooth muscle cell. Role of cyclic-nucleotides and angiotensin1 receptors. J Clin Invest. 1995;96:141–149.
26. Griendling KK, Ushio-Fukai M. Reactive oxygen species as mediators of
angiotensin II signaling. Regul Pept. 2000;91:21–27.
27. Beckman JS, Koppenol WH. Nitric oxide, superoxide, and peroxynitrite:
the good, the bad, and ugly. Am J Physiol. 1996;271:C1424 –C1437.
28. Chillon JM, Baumbach GL. Effects of an angiotensin-converting enzyme
inhibitor and a ␤-Blocker on cerebral arteriolar dilation in hypertensive
rats. Hypertension. 2001;37:1388 –1393.
Cerebral Arteriolar Structure in Mice Overexpressing Human Renin and Angiotensinogen
Gary L. Baumbach, Curt D. Sigmund and Frank M. Faraci
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Hypertension. 2003;41:50-55; originally published online November 4, 2002;
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